1. Field of the Invention
The present invention relates to a circuit module (i.e., substrate), particularly to a mounting structure of a semiconductor IC used in an environment where noise control is difficult, such as high-speed power line communication (PLC) and the like.
2. Description of Related Art
With demand for compact electronic components, numerous IC chips and IC components need to be mounted on a circuit module having semiconductor ICs and IC components. Demand has thus been growing for the effective use of wiring and mounting space. Particularly, a modulation/demodulation IC, which modulates and demodulates signals, is provided with a large number of ground terminals (pads) at a narrow pitch, each of which is connected onto a mounting board. Used as the mounting board is a laminated board, which includes a plurality of laminated wiring layers having an insulating layer in between (e.g., Japanese Patent No. 3,375,555 and Japanese Patent Laid-open Publication No. 2000-031329). In order to minimize wire routing and to reduce impedance attributed to wiring itself, a power line and a ground line included in the wiring layers normally have a planar shape, and are mounted in the laminated board as a power plate and a ground plate respectively.
When such a conventional power plate or ground plate is mounted in the laminated board as an internal layer, determining a particular mounting position is handled as a design matter. When the ground plate that configures ground layer 2200 is provided away from modulation/demodulation IC 2100 as shown in FIG. 19, for instance, a via needs to be provided to a predetermined depth for electrical connection between a ground terminal and the ground plate. It is easy to process via hole (hereinafter referred to as through-via) H that penetrates entire laminated board 1000. However, such a deep through-via requires a large planar space and a space on a rear side, thus wasting the mounting and wiring spaces. Further, a vertical distance cannot be overlooked, and an increase in inductance caused by the increased thickness of the laminated board is not a small problem.
It is possible to provide the via only to the middle in order to secure the mounting space. There is a problem, however, where the farther the ground layer is provided from the ground terminal of modulation/demodulation IC 2100, the more the processing accuracy declines. In addition, the increase in inductance caused by the increased thickness of the laminated board is more than a small problem, as described above.